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F or many years now, it\u2019s been well established that the mass extinction at the end of the Cretaceous period \u2013 which led to the extinction of the dinosaurs as well as the loss of 75 per cent of all plant and animal species on Earth \u2013 was associated with a huge impact. The Chicxulub crater, buried beneath the Yucat\u00e1n Peninsula in Mexico, was identified in 1990 as a giant impact feature \u2013 some 150km across and 20km deep (although by now well filled-in with sediments). The calculated formation of this crater coincides with the mass extinction, around 66 million years ago, and the impactor is believed to have been an asteroid about 10\u201315km in diameter.<\/span><\/p>\n\n The Chicxulub crater is one of the largest impact structures ever found on Earth. But there\u2019s something else exceptional about this impact. It seems that the crater was created by a carbonaceous chondrite (CC) asteroid \u2013a dark rock full of organic compounds. This is surprising because impacts from this sort of object are rare \u2013 they make up only five per cent of all meteorites collected. So why is it that one of the largest craters on Earth \u2013 and the impact that wiped out the dinosaurs \u2013 was formed by such an asteroid?<\/p>\n\n David Nesvorn\u00fd and his colleagues at the Department of Space Studies, Southwest Research Institute, in the US, have been trying to get to the bottom of this mystery. They built a computer simulation of how the orbits of objects in the main asteroid belt can be dislodged to become near-Earth asteroids that have the potential to collide with our planet. Their model included over 42,000 asteroids with widths greater than 5km in the main belt, and how they\u2019re affected by influences such as the radiation pressure of sunlight or Jupiter\u2019s gravity.<\/p>\n\n They calculated that Venus and Earth receive about the same number of strikes from asteroids bigger than 5km, with Mars being hit about three times less. Interestingly, Nesvorn\u00fd found that about six per cent of the simulated Venus impactors had evolved into a retrograde orbit beforehand \u2013 equivalent to cars colliding head-on with a much higher impact speed \u2013 and hit the planet at a staggering 220,000km\/h.<\/p>\n\n But what about the composition of the largest impactors, such as the Chicxulub asteroid? Nesvorn\u00fd and his colleagues found that because of the orbital dynamics involved, smaller impactors (less than a kilometre in diameter) are most likely to have come from the inner edge of the asteroid belt, so have an ordinary, stony composition \u2013 as found with most meteorites discovered. But the largest impactors hitting Earth are more likely to have originated from the middle or outer asteroid belt, where CC objects are more common. They conclude that the impactor that triggered the mass extinction 66 million years ago was a main-belt asteroid that quite likely (they calculate around 60 per cent probability) originated from beyond 2.5 AU.<\/p>\n\nMeasuring impacts<\/strong><\/h4>\n\n
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